ekf2, fw_mode_manager, fw_rate_control: remove unused using declarations

Remove using-declarations for math::constrain, math::max, and
math::min that are never used — all call sites use the fully-qualified
form (e.g. math::constrain()).

Fixes misc-unused-using-decls clang-tidy diagnostic.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

src/modules/fw_rate_control/FixedwingRateControl.cpp

@@ -36,7 +36,6 @@
 using namespace time_literals;
 using namespace matrix;
 
-using math::constrain;
 using math::interpolate;
 using math::radians;