fix(install.sh): don't remove old npm first

The install script will gracefully fail if things don't work.  This is
especially important for versions of npm that won't work in your current
node version.

PR-URL: #3748
Credit: @wraithgar
Close: #3748
Reviewed-by: @isaacs

Author: wraithgar

scripts/install.sh

@@ -162,8 +162,6 @@ cd "$TMP" \
   && curl -qSsL -o npm.tgz "$url" \
   && $tar -xzf npm.tgz \
   && cd "$TMP"/package \
-  && echo "removing existing npm" \
-  && "$node" bin/npm-cli.js rm npm -gf --loglevel=silent \
   && echo "installing npm@$t" \
   && "$node" bin/npm-cli.js install -gf ../npm.tgz \
   && cd "$BACK" \