gdb: remove unused print_command_line and print_command_lines declarations

There is no corresponding definition for print_command_line.

There is already a declaration for print_command_lines in
cli/cli-script.h (the implementation is in cli/cli-script.c).

Change-Id: Ic9e67ed04703306d614383ead14e2b2b059b2a8e
Approved-By: Tom Tromey <tom@tromey.com>