Remove notification settings from appveyor.yml

Notifications can be (and should be) configured on account basis on
the CI web site.  This avoids getting emails to openssl-commits for
personal accounts that also build OpenSSL stuff.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3484)
(cherry picked from commit 7a94f5b0)