gdb/infcmd: remove redundant clear_proceed_status call

In `attach_command`, there is a call to `init_wait_for_inferior`
followed by a call to `clear_proceed_status`.  However,
`init_wait_for_inferior` already calls `clear_proceed_status`.  Remove
the redundant call.

Regression-tested on X86_64 Linux.

gdb/ChangeLog:
2020-10-07  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infcmd.c (attach_command): Remove the redundant call to
	`clear_proceed_status`.