Revert "demos: use -o on glslangValidator to allow parallel make"

This reverts commit 2d83e06b0dacb8b4d1364b816ce10e56829e048a.
Do not want to make this change at this time.
diff --git a/glslang_revision b/glslang_revision
index 0ba895a..565eaa4 100644
--- a/glslang_revision
+++ b/glslang_revision
@@ -1 +1 @@
-3a194f7ba46b747411181736203f9389a5406d81
+e6f7988adec197e0590fb8274b0412811456d3e6