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

package/libs/libpcap/Makefile

@@ -21,7 +21,6 @@ PKG_LICENSE:=BSD-3-Clause
 PKG_LICENSE_FILES:=LICENSE
 
 PKG_ASLR_PIE_REGULAR:=1
-PKG_BUILD_PARALLEL:=1
 
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk

package/libs/mbedtls/Makefile

@@ -16,7 +16,6 @@ PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=https://codeload.github.com/ARMmbed/mbedtls/tar.gz/v$(PKG_VERSION)?
 PKG_HASH:=96257bb03b30300b2f35f861ffe204ed957e9fd0329d80646fe57fc49f589b29
 
-PKG_BUILD_PARALLEL:=1
 PKG_LICENSE:=GPL-2.0-or-later
 PKG_LICENSE_FILES:=gpl-2.0.txt
 PKG_CPE_ID:=cpe:/a:arm:mbed_tls