Help: Drop incorrect versionadded mark from CMAKE_MATCH_<n>

It was added by commit 496ec6036f (Help: Add Sphinx 'versionadded'
directives to each top-level document, 2020-07-06, v3.19.0-rc1~558^2)
because the document happened to be added during the 3.9 release series,
but the variable has existed since the beginning.

Help/variable/CMAKE_MATCH_n.rst

@@ -1,8 +1,6 @@
 CMAKE_MATCH_<n>
 ---------------
 
-.. versionadded:: 3.9
-
 Capture group ``<n>`` matched by the last regular expression, for groups
 0 through 9.  Group 0 is the entire match.  Groups 1 through 9 are the
 subexpressions captured by ``()`` syntax.