Disable mfilterjavac for now.

mfilterjavac is currently causing problems when installing the java grade from
the stage 2 dir.  It can be re-enabled when the following sequence of actions
works correctly:

(1) Build stage 1 using rotd-2013-04-18
(2) Bootcheck (or otherwise build the stage 2 compiler).
(3) cd stage2 && mmake install (with the java grade as one of the grades
    to install).

compiler/options.m:
	Disable the use of mfilterjavac for now.

compiler/mlds_to_java.m:
	Update a comment.