remove dynamic import statements

Importing from paths generated at runtime breaks
parcel, see (parcel-bundler/parcel#4031), or
esbuild, see (evanw/esbuild#480)
By removing the calls to the deprecated global MONGOOSE_DRIVER_PATH this
issue can be resolved.

lib/index.js

@@ -4,16 +4,7 @@
  * Module dependencies.
  */
 
-if (global.MONGOOSE_DRIVER_PATH) {
-  const deprecationWarning = 'The `MONGOOSE_DRIVER_PATH` global property is ' +
-    'deprecated. Use `mongoose.driver.set()` instead.';
-  const setDriver = require('util').deprecate(function() {
-    require('./driver').set(require(global.MONGOOSE_DRIVER_PATH));
-  }, deprecationWarning);
-  setDriver();
-} else {
-  require('./driver').set(require('./drivers/node-mongodb-native'));
-}
+require('./driver').set(require('./drivers/node-mongodb-native'));
 
 const Document = require('./document');
 const Schema = require('./schema');
@@ -732,19 +723,17 @@ Mongoose.prototype.connections;
  * Driver dependent APIs
  */
 
-const driver = global.MONGOOSE_DRIVER_PATH || './drivers/node-mongodb-native';
 
 /*!
  * Connection
  */
-
-const Connection = require(driver + '/connection');
+const Connection = require('./drivers/node-mongodb-native/connection');
 
 /*!
  * Collection
  */
 
-const Collection = require(driver + '/collection');
+const Collection = require('./drivers/node-mongodb-native/collection');
 
 /**
  * The Mongoose Aggregate constructor