t/lib-gpg: drop redundant killing of gpg-agent

t/lib-gpg: drop redundant killing of gpg-agent

In 53fc999306 ("gpg-interface t: extend the existing GPG tests with
GPGSM", 2018-07-20), the gpgconf call which kills gpg-agent was copied
from the existing gpg setup code.

The reason for killing gpg-agent is given in 29ff1f8f74 ("t: lib-gpg:
flush gpg agent on startup", 2017-07-20):

  When running gpg-relevant tests, a gpg-daemon is spawned for each
  GNUPGHOME used. This daemon may stay running after the test and cache
  file descriptors for the trash directories, even after the trash
  directory is removed. This leads to ENOENT errors when attempting to
  create files if tests are run multiple times.

  Add a cleanup script to force flushing the gpg-agent for that GNUPGHOME
  (if any) before setting up the GPG relevant-environment.

Killing gpg-agent once per test is sufficient.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>