optimize media queries

[wpalmer]

sometimes, @media queries are redundant, especially when generated by CSS preprocessors which support nested media queries. eg:

@media screen and (min-width: 768px) and (min-width: 800px) {
...
}

These should be rewritten to remove redundancies:

@media screen and (min-width: 800px) {
...
}

similarly, @media queries are sometimes impossible, eg:

@media screen and (max-width: 767px) and (min-width: 800px) {
...
}

these (and their associated blocks) should be completely removed.