Remove invalid CMake argument

DEPENDS is not valid argument for a custom command with a target, and
prior versions of CMake (v<3.31) silently ignored those. This specific
dependency is already covered as the threadx_demo application itself
depends on the threadx target, so the line has been ignored and can be
safely removed.

Change-Id: If4e73ca8595c1e2fa211537887a0bda48d655439
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>