Help: Remove wrong versionadded for CMAKE_COMPILER_IS_GNU* variables

Commit 496ec6036f (Help: Add Sphinx 'versionadded' directives to each
top-level document, 2020-07-06, v3.19.0-rc1~558^2) added the
versionadded directive to many variables, but it wrongly detected the
commit a6d3f5418c (Help: Clarify documentation of
CMAKE_COMPILER_IS_GNU{CC,CXX,G77}, 2016-09-14, v3.7.0-rc1~120^2~1) as
the origin for these variables. In reality, these variables were
introduced in commit f5d95fb078 (Complete rework of makefile generators
expect trouble, 2002-11-08, v2.4.0~4935).