Bug 1262637 - Remove rust mp4parse sse2 check. r=ajones

This reverts bug 1255869 which added a runtime check for sse2
support to work around sse2 instructions in the rust standard
library targeting i686-pc-windows-msvc.

We now make official builds against i586-pc-windows-msvc which
should omit the sse2 instructions which aren't supported for
all our current users.

media/libstagefright/binding/MP4Metadata.cpp

@@ -8,7 +8,6 @@
 #include "media/stagefright/MediaSource.h"
 #include "media/stagefright/MetaData.h"
 #include "mozilla/Logging.h"
-#include "mozilla/SSE.h"
 #include "mozilla/Telemetry.h"
 #include "mp4_demuxer/MoofParser.h"
 #include "mp4_demuxer/MP4Metadata.h"
@@ -123,20 +122,10 @@ private:
 MP4Metadata::MP4Metadata(Stream* aSource)
  : mStagefright(MakeUnique<MP4MetadataStagefright>(aSource))
 #ifdef MOZ_RUST_MP4PARSE
+ , mRust(MakeUnique<MP4MetadataRust>(aSource))
  , mReportedTelemetry(false)
 #endif
 {
-  // Rust's i686-pc-windows-msvc target assumes sse2 support.
-  // While we can pass -C target-feature=-sse2 to disable this
-  // in our own code, there's still the problem of the standard
-  // library. To work around this for the small number of users
-  // without SSE2 hardware, avoid calling into rust code.
-  // https://bugzilla.mozilla.org/show_bug.cgi?id=1253202
-#if defined(MOZ_RUST_MP4PARSE)
-  if (mozilla::supports_sse2()) {
-    mRust = MakeUnique<MP4MetadataRust>(aSource);
-  }
-#endif
 }
 
 MP4Metadata::~MP4Metadata()