fix: remove unnecessary type assertions

[bjornstar]

The main branch is currently failing due to a tslint rule:

Error: /Users/runner/work/got/got/source/core/options.ts: line 953, col 17, Error - This assertion is unnecessary since it does not change the type of the expression. (@typescript-eslint/no-unnecessary-type-assertion)
Error: /Users/runner/work/got/got/source/core/utils/options-to-url.ts: line 66, col 15, Error - This assertion is unnecessary since it does not change the type of the expression. (@typescript-eslint/no-unnecessary-type-assertion)

So I removed the unnecessary type assertions.

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.