Simplify build.sh

The `mdbook-pandoc` extension is always available when building the book in CI, so we can remove the conditional.

From a discussion in #1704.

.github/workflows/build.sh

@@ -37,9 +37,7 @@ else
 fi
 
 mdbook build -d "$dest_dir"
-if [ -f "$dest_dir/pandoc/pdf/comprehensive-rust.pdf" ]; then
-    mv "$dest_dir/pandoc/pdf/comprehensive-rust.pdf" "$dest_dir/html/"
-fi
+mv "$dest_dir/pandoc/pdf/comprehensive-rust.pdf" "$dest_dir/html/"
 (cd "$dest_dir/exerciser" && zip --recurse-paths ../html/comprehensive-rust-exercises.zip comprehensive-rust-exercises/)
 
 echo "::endgroup::"