Remove dual implication between --turbo and --ignition.

This removes the last remaining dual implications between sets of flags.
Support for this was originally added to support multiple subsequent
calls to {SetFlagFromString} switching a set of flags on and off. Now
that Chrome no longer relies on this behavior we can remove support for
this entirely.

Original CL: https://crrev.com/f774d8c56f00de92614886fc4cb541411eff7aa1

R=rmcilroy@chromium.org
BUG=v8:6408

Change-Id: I5f9db8457c562c0b434ea7d6eca9941c76fe7d19
Reviewed-on: https://chromium-review.googlesource.com/527174Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45782}

src/flag-definitions.h

@@ -19,10 +19,6 @@
 #define DEFINE_NEG_NEG_IMPLICATION(whenflag, thenflag) \
   DEFINE_NEG_VALUE_IMPLICATION(whenflag, thenflag, false)
 
-#define DEFINE_DUAL_IMPLICATION(whenflag, thenflag) \
-  DEFINE_IMPLICATION(whenflag, thenflag)            \
-  DEFINE_NEG_NEG_IMPLICATION(whenflag, thenflag)
-
 // We want to declare the names of the variables for the header file.  Normally
 // this will just be an extern declaration, but for a readonly flag we let the
 // compiler make better optimizations by giving it the value.
@@ -273,8 +269,8 @@ DEFINE_BOOL(future, FUTURE_BOOL,
             "not-too-far future")
 DEFINE_IMPLICATION(future, turbo)
 
-DEFINE_DUAL_IMPLICATION(turbo, ignition)
-DEFINE_DUAL_IMPLICATION(turbo, thin_strings)
+DEFINE_IMPLICATION(turbo, ignition)
+DEFINE_IMPLICATION(turbo, thin_strings)
 
 // Flags for experimental implementation features.
 DEFINE_BOOL(allocation_site_pretenuring, true,