Cleanup: remove --disable-inline and related macros

Cleanup: remove --disable-inline and related macros

With the .cci files gone there is no longer any use of the SQUID_INLINE
macro. Which in turn removes the need for _USE_INLINE_ macro and then the
--disable-inline build option which sets that.

The relevant compiler specific *FLAGS option should be used instead to
disable compiler inlining functions without special macro usage.