gdb/dwarf: remove line_header parameter from dwarf2_decode_lines

The function declaration for dwarf2_decode_lines is:

  void dwarf_decode_lines (struct line_header *lh, struct dwarf2_cu *cu,
                           unrelocated_addr lowpc, bool decode_mapping)

However, it is always the case that:

  lh == cu->line_header

I propose that we simplify the dwarf_decode_lines signature by
removing the line_header parameter.  The same is true for
dwarf_decode_lines_1, which is only called from dwarf_decode_lines.

I'm proposing this change because I was looking through the DWARF
code, trying to understand how symtabs are created, and this extra
complexity just makes things harder to understand: what is the
relationship between the line_header (LH) and dwarf2_cu (CU)
parameters?  When would we ever want to process a line_header other
than the one attached to CU? (answer: never, and we don't).  This
simplification makes things easier to understand (IMHO).
There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>