[Pal/Linux-SGX] Remove redundant memset in _DkVirtualMemoryAlloc()

There is already one memset in get_enclave_pages() controlled by
manifest option `sgx.zero_heap_on_demand`. So the explicit memset
in _DkVirtualMemoryAlloc() is not needed. Observed perf improvement
is 30% on a worst-case micro-benchmark with mmap().

Pal/src/host/Linux-SGX/db_memory.c

@@ -54,8 +54,6 @@ int _DkVirtualMemoryAlloc(void** paddr, uint64_t size, int alloc_type, int prot)
     if (!mem)
         return addr ? -PAL_ERROR_DENIED : -PAL_ERROR_NOMEM;
 
-    memset(mem, 0, size);
-
     *paddr = mem;
     return 0;
 }