Revert "[ptr-compr] Switch Smis to 31-bit on 64-bit platforms."

This reverts commit bf1e47e6.

Reason for revert: chromium:876631

Original change's description:
> [ptr-compr] Switch Smis to 31-bit on 64-bit platforms.
> 
> This is prerequisite for V8 heap pointer compression.
> 
> Bug: v8:7703
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
> Change-Id: I2cdf02bd4cd535beb78a5db5b7cbdf67433a6d16
> Reviewed-on: https://chromium-review.googlesource.com/1181136
> Commit-Queue: Igor Sheludko <ishell@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55269}

TBR=ulan@chromium.org,ishell@chromium.org,verwaest@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7703
Change-Id: I6d8662a6ff0b352e4975d31330d63a705ed55cd5
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1186341Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55333}

include/v8.h

@@ -245,7 +245,7 @@ static_assert(
     "Pointer compression can be enabled only for 64-bit architectures");
 typedef SmiTagging<4> PlatformSmiTagging;
 #else
-typedef SmiTagging<4> PlatformSmiTagging;
+typedef SmiTagging<kApiPointerSize> PlatformSmiTagging;
 #endif
 
 const int kSmiShiftSize = PlatformSmiTagging::kSmiShiftSize;