[binutils-gdb] Remove one explicit rule for monitor.o

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT aa70c9f1950b25b48f8fd6770f5d38a8252c24a4 ***

Author: Yao Qi <yao.qi@linaro.org>
Branch: master
Commit: aa70c9f1950b25b48f8fd6770f5d38a8252c24a4

Remove one explicit rule for monitor.o

gdb/monitor.c was removed by 40e0b27 (Delete the remaining ROM monitor
targets).

gdb:

2017-09-19  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (monitor.o): Remove the rule.