Replace or remove sourcemap support

Sourcemap implementation has a lot of bugs and no contributors. Please only post a comment if you are able to assist with coding.

Problems

• Numerous reports of it "not working" that tend to lack reproducible details
• #299 (closed)
• Reliance on Symbolic Python bindings that no longer exist, which means we can't update symbolic.
• No full unit test for mobile app support
• Code quality is very low - ported from old open source sentry code. Not written in a way that makes sense for how GlitchTip. Written by different people who aren't active anymore. Not a dig on anyone, certainly I'm at fault here.

Removing it would be a last resort. Another option might be to document that it isn't fully supported and most users shouldn't use it.