Erroneous assert was left in swapchain_base.cpp after some refactoring
was done. This commit removes the assert and updates the CMakeLists.txt
file to turn on compiler warnings so that developers can spot these
leftover code earlier.

Change-Id: I7f4051c89a86f21f66f20b0e92f01bc101436c71
Signed-off-by: Dennis Tsiang <dennis.tsiang@arm.com>