MAINTAINERS: intel_pmc_core: Update MAINTAINERS

This removes the entry for pmc_core.h file in the MAINTAINERS as the
file is already removed by a previous commit.
"platform/x86: intel_pmc_core: Remove unused header file"

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>