http2: remove unnecessary asserts in core

options.getTrailers & options.selectPadding will always be a
function by the time the code reaches the assert checks. And
even if not (which seems currently impossible), the assert
failure will be almost the same as calling something that is
not a function. The downside of leaving these is that typeof
checks are decently expensive.

PR-URL: #16327
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: apapirovski

lib/internal/http2/core.js

@@ -218,13 +218,8 @@ function onSessionTrailers(id) {
   assert(stream !== undefined,
          'Internal HTTP/2 Failure. Stream does not exist. Please ' +
          'report this as a bug in Node.js');
-  const getTrailers = stream[kState].getTrailers;
-  // We should not have been able to get here at all if getTrailers
-  // was not a function, and there's no code that could have changed
-  // it between there and here.
-  assert.strictEqual(typeof getTrailers, 'function');
   const trailers = Object.create(null);
-  getTrailers.call(stream, trailers);
+  stream[kState].getTrailers.call(stream, trailers);
   const headersList = mapToHeaders(trailers, assertValidPseudoHeaderTrailer);
   if (!Array.isArray(headersList)) {
     process.nextTick(emit, stream, 'error', headersList);
@@ -382,9 +377,6 @@ function onGoawayData(code, lastStreamID, buf) {
 // maxPayloadLen. The method must return a numeric value within the range
 // frameLen <= n <= maxPayloadLen.
 function onSelectPadding(fn) {
-  assert(typeof fn === 'function',
-         'options.selectPadding must be a function. Please report this as a ' +
-         'bug in Node.js');
   return function getPadding() {
     debug('fetching padding for frame');
     const frameLen = paddingBuffer[PADDING_BUF_FRAME_LENGTH];