GL: remove stray unused expression.

This originated in 7ad928ae42 which looks
like a generic find & replace that removed all early return statements
because the unmasked renderer string on WebGL could expose both the
underlying driver (NVidia, Mesa, ...) and the wrapper (ANGLE, ...).
Ideally the compiler would warn about an unused expression, but
Optional::operator*() isn't const on mutable variables, so it didn't.

Only discovered this because I'm removing the Optional wrapper from this
member, and then the compiler started to warn.

src/Magnum/GL/Implementation/driverSpecific.cpp

@@ -591,8 +591,6 @@ auto Context::detectedDriver() -> DetectedDrivers {
 
         if(renderer.contains("SVGA3D"_s))
             *_detectedDrivers |= DetectedDriver::Svga3D;
-
-        *_detectedDrivers;
     }
 
     if(vendor.contains("NVIDIA Corporation"_s))