Remove NoHooksLU.BuildHooksEnv

Since NoHooksLU defines HPATH as None, BuildHooksEnv will never be called (as
the LogicalUnit.BuildHooksEnv docstring correctly states). Removing the
function altogether, to avoid having dead code lying around, and to make sure
the non-implemented one in LogicalUnit will be called if someone mistakenly
tries to call it, so it's immediately clear that something is wrong.

Reviewed-by: iustinp