v8: remove optimization switches

Remove compiler switches from $(TOPLEVEL)/deps/v8/build/common.gypi, we set
them globally in $(TOPLEVEL)/common.gypi.

Commit 29d12c73 accidentally reintroduced the switches again. In particular,
the 'cflags!': ['-O2','-Os'] section forced building V8 without any
optimizations, resulting in a steep (~66%) performance drop on some benchmarks.

Fixes #4191.