OpenGL: Clean up legacy __ANDROID_VNDK__ in libEGL

libEGL has already been marked as LLNDK, thus the __ANDROID_VNDK__
doesn't make sense inside libEGL anymore.

Test: BOARD_VNDK_VERSION=current m -j libEGL.vendor
Test: BOARD_VNDK_VERSION=current m -j libGLESv2.vendor
Change-Id: Ib96985ad35093b3bd6eb16237f133703bc267fa0