Remove `no-unused-variable` in TSLint config

[saadq]

TSLint version >= 2.9 seems to have deprecated no-unused-variable.

...the changes in 2.9 make it much more complex for this rule to work as intended, especially with respect to the ignorePattern option.

It looks like people should now just make sure that their tsconfig.json has the noUnusedLocals option enabled which covers most of what no-unused-variable did.

Changes would need to be made here.