devel/spirv-llvm-translator: unbreak build by dropping llvm13 flavor

-- SPIR-V Headers location is not specified. Will try to download
          spirv.hpp from https://github.com/KhronosGroup/SPIRV-Headers into
          /wrkdirs/usr/ports/devel/spirv-llvm-translator/work-llvm13/.build/SPIRV-Headers
CMake Error at /usr/local/share/cmake/Modules/ExternalProject.cmake:2650 (message):
  error: could not find git for clone of spirv-headers-populate
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/ExternalProject.cmake:3700 (_ep_add_download_command)
  CMakeLists.txt:22 (ExternalProject_Add)

This reverts commit 1b058e5.

devel/spirv-llvm-translator/Makefile

@@ -1,7 +1,6 @@
 PORTNAME=	spirv-llvm-translator
 DISTVERSIONPREFIX=	v
 DISTVERSION=	${DISTVERSION_${FLAVOR}}
-DISTVERSION_llvm13=	13.0.0
 DISTVERSION_llvm12=	12.0.0
 DISTVERSION_llvm11=	11.0.0
 CATEGORIES=	devel
@@ -15,7 +14,7 @@ LICENSE_FILE=	${WRKSRC}/LICENSE.TXT
 
 LIB_DEPENDS=	libLLVM-${DISTVERSION:R:R}.so:devel/${FLAVOR}
 
-FLAVORS=	${13 12 11:L:S/^/llvm/}
+FLAVORS=	${12 11:L:S/^/llvm/}
 
 USES=		cmake compiler:c++11-lib
 USE_GITHUB=	yes