Revert "[test] Add Liftoff variant"

This reverts commit 712fa675.

Reason for revert: Infra side change was reverted. Please reland
after it stays in for a bot cycle...

Original change's description:
> [test] Add Liftoff variant
> 
> Add a variant for testing the current state of the Liftoff
> implementation.
> This variant will only run on a subset of the bots, just like the
> --future variant.
> 
> R=​machenbach@chromium.org, hablich@chromium.org
> 
> Bug: v8:7088, v8:6600
> Change-Id: If49fad3a8ed579356504b821a787326754f24e78
> Reviewed-on: https://chromium-review.googlesource.com/779420
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49504}

TBR=machenbach@chromium.org,hablich@chromium.org,clemensh@chromium.org

Change-Id: Ib6b2e79cea5d9f99f8933c72bbb9d9dddbd6ae07
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7088, v8:6600
Reviewed-on: https://chromium-review.googlesource.com/779719Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49507}

tools/testrunner/local/variants.py

@@ -6,7 +6,6 @@
 ALL_VARIANT_FLAGS = {
   "default": [[]],
   "future": [["--future"]],
-  "liftoff": [["--liftoff"]],
   "stress": [["--stress-opt", "--always-opt"]],
   # TODO(6792): Write protected code has been temporary added to the below
   # variant until the feature has been enabled (or staged) by default.
@@ -23,7 +22,6 @@ ALL_VARIANT_FLAGS = {
 FAST_VARIANT_FLAGS = {
   "default": [[]],
   "future": [["--future"]],
-  "liftoff": [["--liftoff"]],
   "stress": [["--stress-opt"]],
   # TODO(6792): Write protected code has been temporary added to the below
   # variant until the feature has been enabled (or staged) by default.
@@ -36,6 +34,5 @@ FAST_VARIANT_FLAGS = {
   "wasm_traps": [["--wasm_trap_handler", "--invoke-weak-callbacks"]],
 }
 
-ALL_VARIANTS = set(["default", "future", "liftoff", "stress",
-                    "stress_incremental_marking", "nooptimization",
-                    "stress_background_compile", "wasm_traps"])
+ALL_VARIANTS = set(["default", "future", "stress", "stress_incremental_marking",
+                    "nooptimization", "stress_background_compile", "wasm_traps"])

tools/testrunner/standard_runner.py

@@ -48,7 +48,7 @@ VARIANT_ALIASES = {
   # Additional variants, run on all bots.
   "more": MORE_VARIANTS,
   # Additional variants, run on a subset of bots.
-  "extra": ["future", "liftoff"],
+  "extra": ["future"],
 }
 
 GC_STRESS_FLAGS = ["--gc-interval=500", "--stress-compaction",