Don't force update check on plugin install

This reduces our dependency on WordPress.org API for installing.

The genesis of this call appears to be a copy and paste error:
1.
https://github.com/wp-cli/wp-cli/commit/1e2b9ee2384139c16aadb772ce0ccbcd7df52acb#diff-59ccae364a062cc72d61b0199f525e79R112
2.
https://github.com/wp-cli/wp-cli/commit/1e2b9ee2384139c16aadb772ce0ccbcd7df52acb#diff-78c563be62d40fb1c0c180515eef9636L60

See #1493