Remove unnecessary 'use' forms

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.
auto merge of #6833 : fdr/rust/fix-warnings, r=graydon

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.