Remove unnecessary std::unary_function base classes

The function objects work equally well without them, and the base
classes were wrong for both types:
 * HashForEntry: returns size_t but declared to return hash_t
   (uint32_t)
 * EqualityForHashedEntries: returns bool and takes two parameters but
   declared to return hash_t and take one parameter

std::unary_function was deprecated in C++11 and removed in C++17.
Upstream libc++ now removes the type for new-enough C++ dialects.

Bug: http://b/175635923
Test: treehugger
Change-Id: I2ff15c5da6a4e4f71df08c243f8af2f11d8d2b0d