remove now unused POSIX regex handling

Finally have sufficient compiler support everywhere to remove the POSIX regex support and just use C++ regex.

Previously had a transitional construct:

// Newer compilers support regex directly
#if (_GLIBCXX_RELEASE >= 7) || (__clang_major__ >= 7)
typedef regExpCxx regExp;
#else
typedef regExpPosix regExp;
#endif

which means that we have not actually been using the POSIX version for a while now. Can confirm this with the output from applications/test/regex1 which indicates this type of output:

_GLIBCXX_RELEASE = 7
Foam::regExp uses C++11 regex