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/libs/libjson-c/Makefile

@@ -21,7 +21,6 @@ PKG_LICENSE_FILES:=COPYING
 PKG_CPE_ID:=cpe:/a:json-c_project:json-c
 
 PKG_FLAGS := nonshared
-PKG_BUILD_PARALLEL:=1
 HOST_BUILD_PREFIX:=$(STAGING_DIR_HOST)
 
 include $(INCLUDE_DIR)/package.mk