[SYCL][E2E] Remove fusion test that never runs (#15864)

This test is never ran because of `REQUIRES: fusion`. This feature used
to be auto-detected, but was removed in #15185. This patch also removes
mentions of this feature from the e2e-test readme.

Fixes part of #15838

Author: ayylol

sycl/test-e2e/NewOffloadDriver/diamond_shape.cpp

@@ -229,7 +229,6 @@ environment:
  * **dump_ir**: - compiler can / cannot dump IR;
  * **llvm-spirv** - llvm-spirv tool availability;
  * **llvm-link** - llvm-link tool availability;
- * **fusion**: - Runtime supports kernel fusion;
  * **aspect-\<name\>**: - SYCL aspects supported by a device;
  * **arch-\<name\>** - [SYCL architecture](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/experimental/sycl_ext_oneapi_device_architecture.asciidoc) of a device (e.g. `arch-intel_gpu_pvc`, the name matches what you
    can pass into `-fsycl-targets` compiler flag);