Remove pg_strtouint64(), use strtoull() directly

pg_strtouint64() is a wrapper around strtoull/strtoul/_strtoui64, but it
seems no longer necessary to have this indirection.

msvc/Solution.pm claims HAVE_STRTOULL, so the "MSVC only" part seems
unnecessary. Also, we have code in c.h to substitute alternatives for
strtoull() if not found, and that would appear to cover all currently
supported platforms, so having a further fallback in pg_strtouint64()
seems unnecessary.

(AFAICT, the only buildfarm member that does not have strtoull()
directly but relies on the code in c.h is gaur. So we can hang on to
that code for a while longer, but its utility is also fading away.)

Therefore, remove pg_strtouint64(), and use strtoull() directly in all
call sites.

(This is also useful because we have pg_strtointNN() functions that have
a different API than this pg_strtouintNN(). So removing the latter
makes this problem go away.)