There should be no need for a private SwSubFont copy assignment op

...given that its (implicitly defined) copy ctor is public.  Before
ad355e2b24 "New: Proportional font width" the
class's members were all private, and then were gradually changed to public,
that's probably the reason why the copy assignment op still is.  Making it
public and implicitly declared avoids -Werror=deprecated-copy with GCC trunk
towards GCC 9.

Change-Id: I940c04041b0447f2129aaaa48f19fec40352311e
Reviewed-on: https://gerrit.libreoffice.org/56920
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sw/source/core/inc/swfont.hxx

@@ -64,8 +64,6 @@ class SwSubFont : public SvxFont
                           const sal_uInt16 nOldAscent ) const;
     void CalcEsc( SwDrawTextInfo const & rInf, Point& rPos );
 
-    SwSubFont& operator=( const SwSubFont& );
-
     short CheckKerning_( );
 
     bool ChgFnt( SwViewShell const *pSh, OutputDevice& rOut );

sw/source/core/txtnode/swfont.cxx

@@ -888,8 +888,6 @@ SwFont::~SwFont()
 {
 }
 
-SwSubFont& SwSubFont::operator=( const SwSubFont& ) = default;
-
 SwFont& SwFont::operator=( const SwFont &rFont )
 {
     m_aSub[SwFontScript::Latin] = rFont.m_aSub[SwFontScript::Latin];