configure.ac: Remove swr_llvm_check()

configure.ac: Remove swr_llvm_check()

No need for an additional function here.
Use the same style for LLVM checks as the other drivers
(e.g. r300, llvmpipe) that don't need a load of other checks.
Instead of open conding the LLVM version check, use the
function used by other drivers.

"enable_gallium_llvm" is checked by gallium_require_llvm().

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>