Remove source lines from parser error messages.

This was a gimmick in the first place. It doesn't work well with IO instances
(= requires re-reading of the input), the code was too complex and it wasn't
that useful in the end. Lets just get rid of it.

This fixes #53.

spec/oga/xml/parser/error_spec.rb

@@ -16,32 +16,12 @@ describe Oga::XML::Parser do
       expect { parse(@invalid_xml) }.to raise_error(Racc::ParseError)
     end
 
-    example 'include the offending input when using String as input' do
-      # Racc basically reports errors at the last moment instead of where they
-      # *actually* occur.
-      partial = <<-EOF.strip
-   1. <person>
-   2. <name>Alice</name>
-   3. <age>25
-   4. <nationality>Dutch</nationality>
-=> 5. </person>
-      EOF
-
-      parse_error(@invalid_xml).should =~ /#{partial}/
+    example 'include the line number when using a String as input' do
+      parse_error(@invalid_xml).should =~ /on line 5/
     end
 
-    example 'include the offending input when using IO as input' do
-      # Racc basically reports errors at the last moment instead of where they
-      # *actually* occur.
-      partial = <<-EOF.strip
-   1. <person>
-   2. <name>Alice</name>
-   3. <age>25
-   4. <nationality>Dutch</nationality>
-=> 5. </person>
-      EOF
-
-      parse_error(StringIO.new(@invalid_xml)).should =~ /#{partial}/
+    example 'include the line number when using an IO as input' do
+      parse_error(StringIO.new(@invalid_xml)).should =~ /on line 5/
     end
 
     example 'use more friendly error messages when available' do