gdb: remove duplicate cmd_list_element declarations

For some reason we have two locations where cmd_list_elements are
declared, cli/cli-cmds.h and gdbcmd.h.  Worse still there is
duplication between these two locations.

In this commit I have moved all of the cmd_list_element declarations
from gdbcmd.h into cli/cli-cmds.h and removed the duplicates.

There should be no user visible changes after this commit.