Remove support for UseSHM

Hotspot currently supports two ways to make use of explicit large pages (huge pages):
- UseHugeTLBFS - explicitly mmap() large pages using MAP_HUGETLB
- UseSHM - create a shared memory segment using shmget() and SHM_HUGETLB

Both these requires that large pages are configured and pre-allocated, the number of currently allocated large pages can be seen with:
> sysctl vm.nr_hugepagessee

See the kernel documentation for more information:
https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt

The share memory approach also requires the process to be privileged (have the capability to lock memory) and that the system is configured to allow large shared memory segments. To get the current max value:
> sysctl kernel.shmmax

There are very few if any pros with the USeSHM approach and we should investigate if we can remove support for it.