latex: Drop unnecessary requirement on obsoleted substitutefont

It is used *conditionally* in sphinx/texinputs/sphinxpackagesubstitutefont.sty.

See https://bugzilla.redhat.com/show_bug.cgi?id=2433763#c1 for details,
quoted here in case the reference gets lost in the future:

Tom "spot" Callaway from comment #1:

> python-sphinx [...] has a BuildRequires: tex(substitutefont.sty),
> but that component is no longer upstream.
> Sure enough, sphinx doesn't actually depend on the obsoleted
> substitutefont tex component (anymore) and has created a wrapper
> in sphinx/texinputs/sphinxpackagesubstitutefont.sty.

python-sphinx.spec

@@ -144,7 +144,6 @@ BuildRequires: tex(parskip.sty)
 BuildRequires: tex(pict2e.sty)
 BuildRequires: tex(polyglossia.sty)
 BuildRequires: tex(remreset.sty)
-BuildRequires: tex(substitutefont.sty)
 BuildRequires: tex(tabulary.sty)
 BuildRequires: tex(textalpha.sty)
 BuildRequires: tex(textcomp.sty)
@@ -309,7 +308,6 @@ Requires:      tex(parskip.sty)
 Requires:      tex(pict2e.sty)
 Requires:      tex(polyglossia.sty)
 Requires:      tex(remreset.sty)
-Requires:      tex(substitutefont.sty)
 Requires:      tex(tabulary.sty)
 Requires:      tex(textalpha.sty)
 Requires:      tex(textcomp.sty)