Remove solve_multivariate

	deleted:    sympy/solvers/solve_multivariate.py
	deleted:    sympy/solvers/tests/test_solve_multivariate.py

There has been little progress on mulitvariate solvers and the code in current
state is not very usable so I feel it is better to drop it. The thing that might be
useful in future when someone restarts working on them will be the tests.
In that situation they can be resurrected from the previous commit. So do not
rebase.