Remove unused declarations in SVGFEDropShadowElement

StdDeviationXIdentifier/StdDeviationYIdentifier don't have definitions.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I187cd3d4991e89dc91e6a021771616cd441e0c22
Reviewed-on: https://chromium-review.googlesource.com/992493Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#547716}

third_party/WebKit/Source/core/svg/SVGFEDropShadowElement.h

@@ -51,9 +51,6 @@ class SVGFEDropShadowElement final
   bool SetFilterEffectAttribute(FilterEffect*, const QualifiedName&) override;
   FilterEffect* Build(SVGFilterBuilder*, Filter*) override;
 
-  static const AtomicString& StdDeviationXIdentifier();
-  static const AtomicString& StdDeviationYIdentifier();
-
   Member<SVGAnimatedNumber> dx_;
   Member<SVGAnimatedNumber> dy_;
   Member<SVGAnimatedNumberOptionalNumber> std_deviation_;