VK_KHR_variable_pointers: Delete an unused and invalid variable

VK_KHR_variable_pointers: Delete an unused and invalid variable

The %float_var variable was not being used in any way by the test.  It
was also invalid because it had the StorageBuffer storage class (so it's
in external memory) but also had a constant initializer.  The real
variable being tested in these tests is %f32_ptr_var which has the
Function storage class so a constant initializer is fine.

Component: Vulkan

VK-GL-CTS issue: https://gitlab.khronos.org/Tracker/vk-gl-cts/issues/686

Affects:
dEQP-VK.spirv_assembly.instruction.graphics.variable_pointers.nullptr_graphics.opvariable_initialized_null_*
dEQP-VK.spirv_assembly.instruction.graphics.variable_pointers.nullptr_graphics.opselect_null_or_valid_ptr_*

Change-Id: I9e2f118b1c774a84c0b6900ef0db081cb5256c2f