remove N-API implementation and v8.x support

Remove the files associated with the external implementation of N-API
and the v8.x Travis CI testing. This move is possible because of v8.x
EOL, which means that all supported versions of Node.js now have an
internal implementation of N-API.

Fixes: #463
Fixes: #509

.travis.yml

@@ -12,8 +12,6 @@ env:
     # https://github.com/jasongin/nvs/blob/master/doc/CI.md
     - NVS_VERSION=1.4.2
   matrix:
-    - NODEJS_VERSION=node/6
-    - NODEJS_VERSION=node/8
     - NODEJS_VERSION=node/10
     - NODEJS_VERSION=node/12
     - NODEJS_VERSION=node/13
@@ -22,7 +20,6 @@ matrix:
   fast_finish: true
   allow_failures:
     - env: NODEJS_VERSION=nightly
-    - env: NODEJS_VERSION=node/6
 sudo: false
 cache:
   directories:

index.js

@@ -1,45 +1,8 @@
-var path = require('path');
-
-var versionArray = process.version
-  .substr(1)
-  .replace(/-.*$/, '')
-  .split('.')
-  .map(function(item) {
-    return +item;
-  });
-
-// TODO: Check if the main node semantic version is within multiple ranges,
-// or detect presence of built-in N-API by some other mechanism TBD.
-
-// We know which version of Node.js first shipped the incarnation of the API
-// available in *this* package. So, if we find that the Node.js version is below
-// that, we indicate that the API is missing from Node.js.
-var isNodeApiBuiltin = (
-  versionArray[0] > 8 ||
-  (versionArray[0] == 8 && versionArray[1] >= 6) ||
-  (versionArray[0] == 6 && versionArray[1] >= 15) ||
-  (versionArray[0] == 6 && versionArray[1] >= 14 && versionArray[2] >= 2));
-
-// The flag is not needed when the Node version is not 8, nor if the API is
-// built-in, because we removed the flag at the same time as creating the final
-// incarnation of the built-in API.
-var needsFlag = (!isNodeApiBuiltin && versionArray[0] == 8);
-
-var include = [__dirname];
-var gyp = path.join(__dirname, 'src', 'node_api.gyp');
-
-if (isNodeApiBuiltin) {
-  gyp += ':nothing';
-} else {
-  gyp += ':node-api';
-  include.unshift(path.join(__dirname, 'external-napi'));
-}
+const path = require('path');
 
 module.exports = {
-  include: include.map(function(item) {
-    return '"' + item + '"';
-  }).join(' '),
-  gyp: gyp,
-  isNodeApiBuiltin: isNodeApiBuiltin,
-  needsFlag: needsFlag
+  include: `"${__dirname}"`,
+  gyp: path.join(__dirname, 'src', 'node_api.gyp:nothing'),
+  isNodeApiBuiltin: true,
+  needsFlag: false
 };