When GCC is used (default for Linux), we should prefer the gcc offered
by the gcc formula. As such even if users install a gcc with higher
version from `gcc@*` formula, it will not be picked up to build other
formulae. This would also allow users to safely delete `gcc@*` formula.
Closes#5953.
Deprecate more methods. Internal APIs have been verified to be unused
elsewhere and removed. External APIs have had deprecation methods added.
Existing deprecations have been either upgraded to produce warnings or
no longer deprecated and the reasoning documented.