Revert "Refactor AppHarness to use reflex run subprocess instead of threads (#5555)"

This reverts commit 9c6cb12.

Author: adhami3310

reflex/environment.py

@@ -650,9 +650,6 @@ class EnvironmentVariables:
     # Enable full logging of debug messages to reflex user directory.
     REFLEX_ENABLE_FULL_LOGGING: EnvVar[bool] = env_var(False)
 
-    # The path to the reflex errors log file. If not set, no separate error log will be used.
-    REFLEX_ERROR_LOG_FILE: EnvVar[Path | None] = env_var(None)
-
 
 environment = EnvironmentVariables()