[SYCL] Remove debugging SYCL_THROW_ON_BLOCK env var (#7621)

The reasons for my suggestion to remove this var are:

1. Seems to be unused
2. The behavior is not clearly documented.
3. May indicate either incorrect sycl usage like 2+ host accessors blocking each other or fully correct behavior with host task usage

Author: KseniyaTikhomirova

sycl/doc/EnvironmentVariables.md

@@ -190,7 +190,6 @@ variables in production code.</span>
 | `SYCL_PRINT_EXECUTION_GRAPH` | Described [below](#sycl_print_execution_graph-options) | Print execution graph to DOT text file. |
 | `SYCL_DISABLE_EXECUTION_GRAPH_CLEANUP` | Any(\*) | Disable cleanup of finished command nodes at host-device synchronization points. |
 | `SYCL_DISABLE_POST_ENQUEUE_CLEANUP` | Any(\*) | Disable cleanup of enqueued command nodes during submission. |
-| `SYCL_THROW_ON_BLOCK` | Any(\*) | Throw an exception on attempt to wait for a blocked command.  |
 | `SYCL_DEVICELIB_INHIBIT_NATIVE` | String of device library extensions (separated by a whitespace) | Do not rely on device native support for devicelib extensions listed in this option. |
 | `SYCL_PROGRAM_COMPILE_OPTIONS` | String of valid OpenCL compile options | Override compile options for all programs. |
 | `SYCL_PROGRAM_LINK_OPTIONS` | String of valid OpenCL link options | Override link options for all programs. |

sycl/source/detail/scheduler/commands.cpp

@@ -700,12 +700,6 @@ bool Command::enqueue(EnqueueResultT &EnqueueResult, BlockingT Blocking,
       EnqueueResult = EnqueueResultT(EnqueueResultT::SyclEnqueueBlocked, this);
       return false;
     }
-    static bool ThrowOnBlock = getenv("SYCL_THROW_ON_BLOCK") != nullptr;
-    if (ThrowOnBlock)
-      throw sycl::runtime_error(
-          std::string("Waiting for blocked command. Block reason: ") +
-              std::string(getBlockReason()),
-          PI_ERROR_INVALID_OPERATION);
 
 #ifdef XPTI_ENABLE_INSTRUMENTATION
     // Scoped trace event notifier that emits a barrier begin and barrier end