Bug 1105894 - Let unified compilation ride the train. r=gps

It was conservatively kept out of non-nightly builds[1], but a year later, we
can assume there are no blocking issues to build releases without unified
compilation.

1. https://lists.mozilla.org/pipermail/dev-platform/2013-December/002393.html

configure.in

@@ -3534,10 +3534,6 @@ dnl ========================================================
 dnl Disable compiling sources in unified mode.
 dnl ========================================================
 
-if test -z "$NIGHTLY_BUILD"; then
-    MOZ_DISABLE_UNIFIED_COMPILATION=1
-fi
-
 MOZ_ARG_DISABLE_BOOL(unified-compilation,
 [  --disable-unified-compilation
                           Disable unified compilation of some C/C++ sources],