Stop calling getlined_olay_XFile()

[grencez]

I noticed some bugs when getlined_olay_XFile() is called and chunksz_XFileB() is 1. Best to just start migrating to the new Fildesh library functions since they have better test coverage.

Specifically: Replace with sliceline_FildeshX().

• src/pla.cc
• src/uni/unifile.cc
• src/biring.cc
• Remove XFile entirely!