Fix -Wdeprecated-literal-operator in quat.h.

This also removes an unnecessary macro and fixes a warning
about unbalanced pragmas that appears when opening the file
in a clangd-enabled IDE.

Fix: 427542076
Test: presubmit
Flag: EXEMPT no runtime behavior changes
Change-Id: I137758cc4869afe898803d3d014c63f09714dd90