gdb: remove unused includes in xtensa-tdep.c

gdb: remove unused includes in xtensa-tdep.c

include-what-you-use says that these includes are not necessary in
xtensa-tdep.c:

    ../../../src/binutils-gdb/gdb/xtensa-tdep.c should remove these lines:
    - #include "dis-asm.h"  // lines 29-29
    - #include "dummy-frame.h"  // lines 36-36
    - #include "dwarf2.h"  // lines 37-37
    - #include "dwarf2/loc.h"  // lines 39-39
    - #include "inferior.h"  // lines 30-30
    - #include "objfiles.h"  // lines 25-25
    - #include "remote.h"  // lines 45-45
    - #include "serial.h"  // lines 46-46
    - #include "symfile.h"  // lines 24-24

That looks about right, so remove them.  Tested by re-building.

gdb/ChangeLog:

	* xtensa-tdep.c: Remove includes.

Change-Id: I9774ec59a68dd94e06967713d2f271b1760f6e6a