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).

Help/variable/CMAKE_COMPILER_IS_GNUCC.rst

@@ -1,7 +1,5 @@
 CMAKE_COMPILER_IS_GNUCC
 -----------------------
 
-.. versionadded:: 3.7
-
 True if the ``C`` compiler is GNU.
 Use :variable:`CMAKE_C_COMPILER_ID <CMAKE_<LANG>_COMPILER_ID>` instead.

Help/variable/CMAKE_COMPILER_IS_GNUCXX.rst

@@ -1,7 +1,5 @@
 CMAKE_COMPILER_IS_GNUCXX
 ------------------------
 
-.. versionadded:: 3.7
-
 True if the C++ (``CXX``) compiler is GNU.
 Use :variable:`CMAKE_CXX_COMPILER_ID <CMAKE_<LANG>_COMPILER_ID>` instead.

Help/variable/CMAKE_COMPILER_IS_GNUG77.rst

@@ -1,7 +1,5 @@
 CMAKE_COMPILER_IS_GNUG77
 ------------------------
 
-.. versionadded:: 3.7
-
 True if the ``Fortran`` compiler is GNU.
 Use :variable:`CMAKE_Fortran_COMPILER_ID <CMAKE_<LANG>_COMPILER_ID>` instead.