matrices: Remove __mathml__

This was introduced in 5009295.
There is no need for this to exist, matrices are handled by `print_mathml`.

Furthermore, this method would never have worked, except for matrices containing only other matrices - `self[i, j].__mathml__()` would always raise `AttributeError`.

bin/coverage_report.py

@@ -70,7 +70,6 @@ def make_report(
     cov = coverage.coverage()
     cov.exclude("raise NotImplementedError")
     cov.exclude("def canonize")  # this should be "@decorated"
-    cov.exclude("def __mathml__")
     if use_cache:
         cov.load()
     else:

sympy/matrices/matrices.py

@@ -776,15 +776,6 @@ def __len__(self):
         """
         return self.rows * self.cols
 
-    def __mathml__(self):
-        mml = ""
-        for i in range(self.rows):
-            mml += "<matrixrow>"
-            for j in range(self.cols):
-                mml += self[i, j].__mathml__()
-            mml += "</matrixrow>"
-        return "<matrix>" + mml + "</matrix>"
-
     def _matrix_pow_by_jordan_blocks(self, num):
         from sympy.matrices import diag, MutableMatrix
         from sympy import binomial