Remove SVGTextMetrics::m_glyph

This field was previously used for ligature and kerning lookups for SVG
fonts. Since SVGTextMetrics::glyph() is no longer called we can remove the
field and collapse some related code.

BUG=242735

Review URL: https://codereview.chromium.org/665253003

git-svn-id: svn://svn.chromium.org/blink/trunk@184376 bbb929c8-8fbe-4397-9dbb-9b2b20218538