Remove bad "Session" link in User activity logs

Summary:
Fixes T12215. Two issues:

• We build this $session link out of $ip, which is (a) wrong even if $ip was the IP and (b) super wrong since $ip is a tag.
• These links don't work even if we'd built them right: searching by the prefix of a session identifier does nothing.

At least for now, just get rid of the links rather than trying to make this behavior work.

Test Plan:
On People > Activity logs:

• Before patch: Saw bad links with bogus targets in "session" column.
• After patch: Saw plain text in "session" column.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12215

Differential Revision: https://secure.phabricator.com/D17316