[DebugInfo] Remove redundant checks for past-the-end of prologue

The V5 directory and filename tables had checks in to make sure we
hadn't read past the end of the line table prologue. Since previous
changes to the data extractor class ensure we never read past the end,
these checks are now redundant, so this patch removes them.

There is still a check to show that the whole prologue remains within
the prologue length.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D71768