doc/ffprobe.xsd: drop build_date and build_time from programVersionType

Update schema after commit:
103e4c5863

doc/ffprobe.xsd

@@ -254,8 +254,6 @@
     <xsd:complexType name="programVersionType">
       <xsd:attribute name="version"          type="xsd:string" use="required"/>
       <xsd:attribute name="copyright"        type="xsd:string" use="required"/>
-      <xsd:attribute name="build_date"       type="xsd:string" use="required"/>
-      <xsd:attribute name="build_time"       type="xsd:string" use="required"/>
       <xsd:attribute name="compiler_ident"   type="xsd:string" use="required"/>
       <xsd:attribute name="configuration"    type="xsd:string" use="required"/>
     </xsd:complexType>