graphics/mesa-devel: unpin LLVM_VERSION after 29d855b

Similar to lang/clover. Mixing libclc + llvm15 and rusticl + llvm18
also doesn't break Rusticl on my Intel iGPU.

See also https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility

graphics/mesa-devel/Makefile

@@ -120,7 +120,6 @@ OPENCL_MESON_FALSE=	b_lundef # environ
 OPENCL_MESON_TRUE=	gallium-rusticl
 OPENCL_MESON_ON=	-Dgallium-opencl=icd
 OPENCL_MESON_OFF=	-Dgallium-opencl=disabled
-OPENCL_VARS=		LLVM_USES=llvm:15,lib,noexport # chase libclc + rust (LTO)
 OPENCL_VARS+=		COINST_RUN_DEPENDS+=clover>0:lang/clover # ICD
 OPENCL_IMPLIES=		LLVM ${"${PORT_OPTIONS:Miris}":?iris:radeonsi} # any from OPTIONS_GROUP_GALLIUM