Revert "DOC: added documentaiton on how to use custom transformers"

This reverts commit 224730bc.

src/doc/converters/transform_functions.rst

@@ -70,88 +70,3 @@ the usual ``$`` notation:
 There are a number of transform functions that are defined by default (see
 ``src/caoscrawler/default_transformers.yml``). You can define custom transform functions by adding
 them to the cfood definition (see :doc:`CFood Documentation<../cfood>`).
-
-
-Custom Transformers
-===================
-
-Custom transformers are basically python functions having a special form/signature. They need to
-be registered in the cfood definition in order to be available during the scanning process.
-
-Let's assume we want to implement a transformer that replaces all occurrences of single letters
-in the value of a variable with a different letter each. So passing "abc" as `in_letters` and
-"xyz" as `out_letters` would result in a replacement of a value of "scan started" to
-"szxn stxrted". We could implement this in python using the
-following code:
-
-.. code-block:: python
-
-	def replace_letters(in_value: Any, in_parameters: dict) -> Any:
-      """
-      Replace letters in variables
-      """
-
-      # The arguments to the transformer (as given by the definition in the cfood)
-		  # are contained in `in_parameters`. We need to make sure they are set or
-		  # set their defaults otherwise:
-
-      if "in_letter" not in in_parameters:
-          raise RuntimeError("Parameter `in_letters` missing.")
-
-      if "out_letter" not in in_parameters:
-          raise RuntimeError("Parameter `out_letters` missing.")
-
-      l_in = in_parameters["in_letters"]
-      l_out = in_parameters["out_letters"]
-
-
-      if len(l_in) != len(l_out):
-          raise RuntimeError("`in_letters` and `out_letters` must have the same length.")
-
-      for l1, l2 in zip(l_in, l_out):
-          in_value = in_value.replace(l1, l2)
-
-      return in_value
-
-
-This code needs to be put into a module that can be found during runtime of the crawler.
-One possibility is to install the package into the same virtual environment that is used
-to run the crawler.
-
-In the cfood the transfomer needs to be registered:
-
-.. code-block:: yaml
-
-  ---
-  metadata:
-    crawler-version: 0.10.2
-    macros:
-  ---
-  Converters:  # put custom converters here
-  Transformers:
-    replace_letters:  # This name will be made available in the cfood
-      function: replace_letters
-      package: utilities.replace_letters
-
-This would assume that the code for the function `replace_letters` is residing in a file
-called `replace_letters.py` that is stored in a package called `utilities`.
-
-The transformer can then be used in a converter, e.g.:
-
-
-.. code-block:: yaml
-
-  Experiment:
-		type: Dict
-		match: ".*"
-		transform:
-			replace_letters:
-				in: $a
-				out: $b
-				functions:
-				- replace_letters:  # This is the name of our custom transformer
-			      in_letters: "abc"
-			      out_letters: "xyz"
-		records:
-			Report:
-				tags: $b