LogicalUnit.BuildHooksEnv, update docstring

The LogicalUnit.BuildHooksEnv docstring used to say that the node list should
not include the master node. This is obviously not the case checking the
relevant code, and double-checking with iustin he confirmed it just document
some obsolete behaviour. This patch removes those remarks making the docstring
shorter, easier, and, last but not least, correct! ;)

Reviewed-by: iustinp