Make std.file.deleteme @safe

This pull request makes a private function `std.file.deleteme` safe.

`deleteme` is used in several unittests in `std.file`, so this request helps to make them safe.

In this request, I use `std.process.thisProcessID` instead of `core.thread.getpid()`
because they have the same effect and `thisProcessID` is a trusted function.

Note: We cannot remove `import core.thread;` in line 43 because it is used in the unittests for `getTimes`, `getTimesWin` and `dirEntry`.