remove useless returns

Here's a silly insomnia-inspired little patch that removes useless
"return;" lines from some routines.  (I left some alone, because they
seemed to be there more for documentation purposes, such as the ones in
from_char_set_mode and from_char_set_int; also regcomp.c since the
pedigree there is unclear.)

This seems pretty uncontroversial, so I'm not thinking of waiting over
the US holidays to get opinions about it ...

-- 
Álvaro Herrera                PostgreSQL Expert, https://www.2ndQuadrant.com/