clean: remove partial.ts as this is built into TS (#322)

- at least as of TS 2.1: https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype
  - we have a peerDep on TS >=2.4, so should definitely be compatible
  - and TS is on ~4.6 at this point, so that's _really_ old

- remove the file and the declarations and declaration maps
  - don't rebuild as that's usually done as a separate commit

src/index.ts

@@ -7,7 +7,6 @@ import * as tsTypes from "typescript";
 import * as resolve from "resolve";
 import * as _ from "lodash";
 import { IOptions } from "./ioptions";
-import { Partial } from "./partial";
 import { parseTsConfig } from "./parse-tsconfig";
 import { printDiagnostics } from "./print-diagnostics";
 import { TSLIB, TSLIB_VIRTUAL, tslibSource, tslibVersion } from "./tslib";