fix: remove _deno_unstable.ts

[iuioiua]

Sorry, re-create of #2592.

[kt3k]

OK. Let's try this. LGTM

[kt3k]

Note:

• _deno_unstable.ts was a workaround for avoiding showing type error at unstable APIs, even when the user invoked deno without --unstable flag. fix: Bypass TS errors for missing --unstable #1819
• In deno v1.22 we removed default full type checking, and the above issue was gone. https://deno.com/blog/v1.22
• So it now should be ok to remove _deno_unstable.ts

[kt3k]

@iuioiua Thank you for suggesting this!