remove unnecessary import, singularity check

if the matrix `U` is singular, the process will still result in a matrix but it will fail to have the expected property, namely `B*B.H == U`.

src/sage/matrix/matrix2.pyx

@@ -12988,14 +12988,10 @@ cdef class Matrix(Matrix1):
             True
         """
         from sage.matrix.constructor import identity_matrix
-        from sage.misc.functional import sqrt
 
         if not self.is_hermitian():
             raise ValueError("matrix is not Hermitian")
 
-        if self.is_singular():
-            raise ValueError("matrix must be nonsingular")
-
         F = self._base_ring
         n = self.nrows()