Linker error due to multiple instances of variable disable_flushes

[wgolab]

The global variable [disable_flushes[(https://github.com/cmuparlay/flit/blob/main/include/persist/pmem_utils.hpp#L21) is defined in a header file. This causes a linker error if the header is included from multiple cpp files.

[Hao-Wei]

Removed disable_flushes as it was not needed anywhere.