Keep the description in one place, just refer it

Delete the duplicated file description and refer to the original one
in generate_bignum_tests.py.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>

tests/scripts/generate_ecp_tests.py

@@ -1,41 +1,8 @@
 #!/usr/bin/env python3
 """Generate test data for ecp functions.
 
-With no arguments, generate all test data. With non-option arguments,
-generate only the specified files.
-
-Class structure:
-
-Child classes of test_data_generation.BaseTarget (file targets) represent an output
-file. These indicate where test cases will be written to, for all subclasses of
-this target. Multiple file targets should not reuse a `target_basename`.
-
-Each subclass derived from a file target can either be:
-  - A concrete class, representing a test function, which generates test cases.
-  - An abstract class containing shared methods and attributes, not associated
-        with a test function.
-
-Both concrete and abstract subclasses can be derived from, to implement
-additional test cases (see BignumCmp and BignumCmpAbs for examples of deriving
-from abstract and concrete classes).
-
-
-Adding test case generation for a function:
-
-A subclass representing the test function should be added, deriving from a
-file target such as BignumTarget. This test class must set/implement the
-following:
-  - test_function: the function name from the associated .function file.
-  - test_name: a descriptive name or brief summary to refer to the test
-        function.
-  - arguments(): a method to generate the list of arguments required for the
-        test_function.
-  - generate_function_tests(): a method to generate TestCases for the function.
-        This should create instances of the class with required input data, and
-        call `.create_test_case()` to yield the TestCase.
-
-Additional details and other attributes/methods are given in the documentation
-of BaseTarget in test_data_generation.py.
+The command line usage, class structure and available methods are the same
+as in generate_bignum_tests.py.
 """
 
 # Copyright The Mbed TLS Contributors