Remove alpine isnan/isinf workaround

I believe this was fixed with ruby/ruby@74f94b3
https://bugs.ruby-lang.org/issues/15595

This was shipped with Ruby 2.7, so it should be unnecessary now

Author: Earlopain

3.1/alpine3.19/Dockerfile

@@ -215,9 +215,6 @@ RUN set -eux; \
 	patch -p1 -i thread-stack-fix.patch; \
 	rm thread-stack-fix.patch; \
 	\
-# the configure script does not detect isnan/isinf as macros
-	export ac_cv_func_isnan=yes ac_cv_func_isinf=yes; \
-	\
 {{ ) else "" end -}}
 # hack in "ENABLE_PATH_CHECK" disabling to suppress:
 #   warning: Insecure world writable dir