[OpenCL] Remove pragma requirement from Arm dot extension.

This removed the pointless need for extension pragma since
it doesn't disable anything properly and it doesn't need to
enable anything that is not possible to disable.

The change doesn't break existing kernels since it allows to
compile more cases i.e. without pragma statements but the
pragma continues to be accepted.

Differential Revision: https://reviews.llvm.org/D100985