Remove stretch shader from flutter/material and material_ui package

After this pr https://github.com/flutter/flutter/pull/187960, the flutter tool will grab shader file from the new widget library location. The file in material library will no longer be used, we should remove them https://github.com/flutter/flutter/blob/cf2c79135685c854b56051359fcb375188d5d91e/packages/flutter/lib/src/material/shaders/stretch_effect.frag#L4

Review PR 187960 and the existing file at packages/flutter/lib/src/material/shaders/stretch_effect.frag to confirm the new widget library location. Locate the corresponding stretch shader in the material_ui package, remove the obsolete copies, and verify that the flutter tool no longer references them.

Written by the indexing model from the issue text.