gdb: the extra_string in a dprintf breakpoint is never nullptr

Given the changes in the previous couple of commits, this commit
cleans up some of the asserts and 'if' checks related to the
extra_string within a dprintf breakpoint.

This commit:

  1. Adds some asserts to update_dprintf_command_list about the
  breakpoint type, and that the extra_string is not nullptr,

  2. Given that we know extra_string is not nullptr (this is enforced
  when the breakpoint is created), we can simplify
  code_breakpoint::code_breakpoint -- it no longer needs to check for
  the extra_string is nullptr case,

  3. In dprintf_breakpoint::re_set we can remove the assert (this will
  be checked within update_dprintf_command_list, we can also remove
  the redundant 'if' check.

There should be no user visible changes after this commit.