treewide: remove BUILD_PARALLEL from CMake packages

It's already default. The only exception is mt76 which has Ninja
disabled.

Found with:

git grep BUILD_PARALLEL | cut -d ':' -f 1 | sort -u > par
git grep cmake.mk | cut -d ':' -f 1 > cmake
comm -1 -2 par cmake

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/system/rpcd/Makefile

@@ -20,7 +20,6 @@ PKG_MIRROR_HASH:=69d9588a5a910676252eabcd2815af318806cae8316777e8b6eb00b26bdc42c
 PKG_LICENSE:=ISC
 PKG_LICENSE_FILES:=
 
-PKG_BUILD_PARALLEL:=1
 PKG_ASLR_PIE_REGULAR:=1
 
 include $(INCLUDE_DIR)/package.mk