Remove info_terminal_command declaration, make definition static

Remove info_terminal_command declaration, make definition static

The info_terminal_command declaration in inflow.h does not match the
current definition.  It is not needed anyway, as info_terminal_command
is only used locally, so remove it and make the definition static.

gdb/ChangeLog:

	* inferior.h (info_terminal_command): Remove declaration.
	* inflow.c (info_terminal_command): Make static.

Change-Id: I22c3fcc44244e3cf877b5e27eff189af11c39503