gdb/dwarf: remove m_line_header from lnp_state_machine class

gdb/dwarf: remove m_line_header from lnp_state_machine class

Following on from the previous commit, this commit remove
m_line_header from the lnp_state_machine class.  The lnp_state_machine
class already holds m_cu, a dwarf2_cu, and the m_line_header was
always just m_cu->line_header, so instead of holding both of these
separately, lets just hold m_cu, and access the line header through
that.

There should be no user visible changes after this commit.

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