support/testing: test_syslinux: remove BR2_TOOLCHAIN_EXTERNAL_LOCALE

support/testing: test_syslinux: remove BR2_TOOLCHAIN_EXTERNAL_LOCALE

The BR2_TOOLCHAIN_EXTERNAL_LOCALE is only defined for uClibc based
custom external toolchains.

The test_syslinux is using a glibc toolchain, so the
"# BR2_TOOLCHAIN_EXTERNAL_LOCALE is not set" line disapear from
the generated dot config (.config) leading to an error due to
a new check in the testsuite infra.

There is the same problem with:
BR2_TOOLCHAIN_EXTERNAL_HAS_THREADS_DEBUG=y
BR2_TOOLCHAIN_EXTERNAL_HAS_THREADS=y
BR2_TOOLCHAIN_EXTERNAL_HAS_THREADS_NPTL=y

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>