remove symlink support from the updater

Our updater patch (legacy/trac#4234 (moved)) adds support for handling symlinks during MAR file generation and in the updater itself. The original reason for adding this feature was to support meek's use of a second browser for its HTTP tunnel; see legacy/trac#12647 (moved).

We no longer use symlinks on any platform. Kathy and I think we should remove the symlink portions of the legacy/trac#4234 (moved) patch (smaller patches == good).