Remove google sparse/dense hash code.

Replace the single place where it was used with a simple QSet.
Also cleanup the code, as this is not a hot-path thus the performance
is really not that important here.

Comparing before/after showed that DUContext::imports is roughly being
called 10k times when importing GammaRay as a project with a clean
cache. Parsing takes ca. 40s, of which 0.08s where spent in ::imports
with the old code. The new simpler QSet based variant takes ca. 0.1s
which is just as fine - esp. if we can get rid of so much code!