Remove SpdyHash, SpdyHashSet, SpdyHashMap, QuicheHashStringPair.

absl::flat_hash_set, absl::flat_hash_map, and absl::Hash are already used
elsewhere is QUICHE.

SpdyHash is not used.

SpdyHashSetImpl and SpdyHashMapImpl are already absl::flat_hash_set and
absl::flat_hash_map in internal code.  They were std::unordered_set and
std::unordered_map in Chromium, but absl hash containers are better according
to https://abseil.io/tips/136.

QuicheHashStringPair is already absl::Hash in Chromium.  Internal code uses a
custom implementation that is officially deprecated in favor of absl::Hash.

PiperOrigin-RevId: 363562364
Change-Id: I3367a242ac376df7cec4662b3d7967d323048806