Make --warn-unused-imports the default for the Mercury system.

browser/MDB_FLAGS.in:
compiler/COMP_FLAGS.in:
deep_profiler/DEEP_FLAGS.in:
library/LIB_FLAGS.in:
mdbcomp/MDBCOMP_FLAGS.in:
mfilterjavac/MFILTERJAVAC_FLAGS.in:
profiler/PROF_FLAGS.in:
slice/SLICE_FLAGS.in:
ssdb/SSDB_FLAGS.in:
    Add --warn-unused-imports to the list of default compiler flags
    for each directory.

compiler/ml_tailcall.m:
library/io.m:
    Delete some unused imports.

mfilterjavac/MFILTERJAVAC_FLAGS.in

@@ -2,6 +2,7 @@
 --no-infer-all
 --warn-known-bad-format-calls
 --warn-unknown-format-calls
+--warn-unused-imports
 --halt-at-warn
 --no-warn-inferred-erroneous
 --no-mercury-stdlib-dir