gdb: Remove unused extra_lines variable

Clang generates a warning if there is a variable that is set but not used
otherwise ("-Wunused-but-set-variable").  On the default configuration, it
causes a build failure (unless "--disable-werror" is specified).

The only extra_lines use in arrange_linetable function is removed on the
commit 558802e4d1c5dcbd0df7d2c6ef62a6deac247a2f
("gdb: change subfile::line_vector to an std::vector").  So, this variable
should be removed to prevent a build failure.