Remove "Replace existing attachment of the same name" option when adding Trac ticket attachments

[raamdev]

During Boston WordCamp Contributor Day, Nacin mentioned that the option to "Replace existing attachment of the same name" when adding attachments to Trac tickets should be removed or hidden, as that option should never be used (we always want to keep the previously attached files).

[raamdev]

Screenshot showing "Replace existing attachment of the same name" option

[kpdesign]

I believe it should be hidden completely from regular Trac users, but show for anyone who has Trac gardening privs, and unchecked by default for them.

[SergeyBiryukov]

I think it's okay to use it for some quick and small changes right after a patch has been uploaded (like fixing a typo), as long as the previous patch hasn't been reviewed by anyone yet.

I'm fine with hiding it for regular users, but I'd prefer to keep it for Trac gardeners.

[nacin]

In 178:

Trac: Remove 'Replace existing attachment of the same name' for non-bug gardeners.

fixes #220.