support/testing: test_tmux: remove BR2_GENERATE_LOCALE

support/testing: test_tmux: remove BR2_GENERATE_LOCALE

The BR2_GENERATE_LOCALE can only be used by uClibc based toolchains with the
internal toolchain backend [1].

The test_tmux is using a external uClibc toolchain, so the
"BR2_GENERATE_LOCALE="en_US.UTF-8"" line disapear from
the generated dot config (.config) leading to an error due to
a new check in the testsuite infra.

[1] https://git.buildroot.net/buildroot/commit/?id=bd0ffe2206fbd32baf7f4a1dc5fde81cfad70462

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>