build: Remove tests for _pwrite() and __pwrite()

These date back to e1919611 s3-waf: more configure tests and minor fixes in 2010 and were not used at that time and now.

More of these should be removed but the __pwrite() test caused a timeout on an ARM builder in Debian. It might just be a fluke but the tests are pointless regardless.

See also 16eb8005 build: Remove tests for getdents() and getdirentries()