Remove VARLENA_FIXED_SIZE hack, which is irreversibly broken now that

both MULTIBYTE and TOAST prevent char(n) from being truly fixed-size.
Simplify and speed up fastgetattr() and index_getattr() macros by
eliminating special cases for attnum=1.  It's just as fast to handle
the first attribute by presetting its attcacheoff to zero; so do that
instead when loading the tupledesc in relcache.c.