fix: not allow process custom file (fix #1879)

[JuniorTour]

	Before	After
yarn test

• Fix Versions of vue-loader 16.6.0+ issues compile time error while using with vue-svg-loader #1879
• Fix Not allow to process .md files as of v16.5.0 when chaining with markdown loader #1883
• Fix Can this loader be applied to. md files #1890

[haoqunjiang]

Thanks for the help.

The tests are passing.

But the previous code comment says that it fixed an issue that ts-loader is inappropriately applied to non-vue files.
So I need some additional time to confirm that deleting these lines doesn't cause new bugs.

[haoqunjiang]

I've tested it and didn't find regressions. So I think it's good to go.

A safer fix is to replace !/\.vue(\.html)?$/.test with /\.[jt]sx?$/.test. But now that I didn't encounter issues by deleting these statements, I'll go on with this fix, and only add them back when there are specific bug reports.