Do not munmap in MmapFile::~MmapFile

Having any destructor with a global variable in bionic is causing
some issues. Since we don't actually need to munmap in this case, we
remove the destructor to work around that issue.

A small class is used to still munmap during tests.

Bug: 73485611
Test: bionic unit tests
Change-Id: Ibcd45e9b1ab22d187ecfc2738bb87244250d81ea