Remove multiple inheritance from "unintesting call" mock classes.

Internal CL 156157936, which was published in commit
fe402c27790ff1cc9a7e17c5d0aea4ebe7fd8a71, introduced undefined behavior
by casting a base class (internal::{Naggy,Nice,Strict}Base<MockClass>,
using the curiously recurring template pattern) pointer to a derived
class ({Naggy,Nice,Strict}Mock<MockClass>), in the base class'
constructor. At that point, the object isn't guaranteed to have taken on
the shape of the derived class, and casting is undefined behavior.

The undefined behavior was caught by Chrome's CFI build bot [1], and
prevents rolling googletest past that commit / CL.

This commit simplifies the {Naggy,Nice,Strict}Mock class hierarchy in
a way that removes the undefined behavior.

[1] https://www.chromium.org/developers/testing/control-flow-integrity