Remove install script, building from source is now opt-in

The vast majority of people don't need this. Package managers
are (correctly) starting to block install scripts by default,
so this change should reduce install-time warnings.

.gitignore

@@ -1,7 +1,6 @@
 src/build
 src/node_modules
 node_modules
-/coverage
 npm/*/*
 !npm/*/package.json
 test/bench/node_modules
@@ -9,7 +8,6 @@ test/fixtures/output*
 test/fixtures/vips-properties.xml
 test/leak/libvips.supp
 .DS_Store
-.nyc_output
 .vscode/
 package-lock.json
 .idea