This switches to using the CMake FetchContent path to dynamically download
and build PCRE2, allowing us to drop the vendored sources.
The FISH_USE_SYSTEM_PCRE2 CMake option is kept, but if false it now means
fetch-and-build PCRE2 rather than building vendored sources.
Note FetchContent was introduced in CMake 3.11. That is now a prerequisite
for building fish with FISH_USE_SYSTEM_PCRE2 disabled.

Now that PCRE2 is dynamically fetched and built, we can remove the vendored
directory.
Fixes fish-shell#8355

This ensures we don't link against a system installed libpcre2.
Comment in the script why not.

CMake's FetchContent package will check out a git repo and leave
permissions as read-only, causing rm to fail. Pass -f so that rm will
succeed.

We no longer vendor PCRE2 sources, instead we fetch them from the
official repo.