Remove deprecated function LALOpenDataFile()

- replace with either fopen() (in tests) or LALFopen() (everywhere else)
  with mode "rb" (for SFTs) or "r" (everywhere else)
- in XLALFindFiles(), remove prefixing of relative file names with './'
  to avoid LAL_DATA_PATH look-up; this is no longer necessary
- no longer set LAL_DATA_PATH in environment scripts and pkg-config files,
  but do set LAL..._DATADIR in environment scripts to $(pkgdatadir)
- refs #1676