Optimize example/jsonptr string conversion

On a mid-range x86_64 laptop, processing the 181 MiB citylots.json file
from github.com/zemirco/sf-city-lots-json:

$ time gen/bin/example-jsonptr < citylots.json > /dev/null
Before this commit:
real    0m1.687s
After:
real    0m1.677s
Ratio: 1.01x
diff --git a/example/jsonptr/jsonptr.cc b/example/jsonptr/jsonptr.cc
index b355a10..c45a797 100644
--- a/example/jsonptr/jsonptr.cc
+++ b/example/jsonptr/jsonptr.cc
@@ -1199,14 +1199,9 @@
                                    g_query.is_at(g_depth));
         }
 
-        if (vbd & WUFFS_BASE__TOKEN__VBD__STRING__CONVERT_0_DST_1_SRC_DROP) {
-          // No-op.
-        } else if (vbd &
-                   WUFFS_BASE__TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY) {
+        if (vbd & WUFFS_BASE__TOKEN__VBD__STRING__CONVERT_1_DST_1_SRC_COPY) {
           TRY(write_dst(tok.ptr, tok.len));
           g_query.incremental_match_slice(tok.ptr, tok.len);
-        } else {
-          return "main: internal error: unexpected string-token conversion";
         }
 
         if (t.continued()) {