Removed User Mailing List From the Issues Template
Removed User Mailing List From the Issues Template

As discussed in the pull request (#315), the user mailing
list has been removed to prevent the bifurcation of the
user base.

It is preferable to have all user questions, feature
requests, and bug reports filed here via the GitHub issues
feature.

Discussion: https://github.com/libcheck/check/pull/315#discussion_r515748364

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