maint: remove unneeded double quotes on RHS of shell assignments

maint: remove unneeded double quotes on RHS of shell assignments

Run this command:
  git grep -l 'LC_[A-Z]*="' \
    | xargs perl -pi -e 's/(LC_[A-Z]*)="(.*?)"/$1=$2/'
* src/Makefile.am: Write LC_ALL=$$locale, not LC_ALL="$$locale".
* src/date.c (main): Similar, in a comment.
* tests/misc/sort-month: Write LC_ALL=$LOC, not LC_ALL="$LOC".