cxl: remove CONFIG_CXL_PMU entry in drivers/cxl/Kconfig

Commit 5d7107c7 ("perf: CXL Performance Monitoring Unit driver")
added the config entries for CXL_PMU in drivers/cxl/Kconfig and
drivers/perf/Kconfig, so it can be toggled from multiple locations:

[1] Device Drivers
     -> PCI support
       -> CXL (Compute Expres Link) Devices
         -> CXL Performance Monitoring Unit

[2] Device Drivers
     -> Performance monitor support
       -> CXL Performance Monitoring Unit

This complicates things, and nobody else does this.

I kept the one in drivers/perf/Kconfig because CONFIG_CXL_PMU controls
the compilation of drivers/perf/cxl_pmu.c.

Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>