You have a 4/32 MB device. See the following links for further information and what you can do now.
Removing packages that are part of the OpenWrt image does not save you any space.
Only removing packages that have been installed after flashing the images will actually gain some space.
You would be better of building your own image with curl included (if it will fit in the first place). See above how to build an image for 4MB devices.
You would be even better off buying a device with sufficient space for current OpenWrt.
You did not really say what you are trying to accomplish with curl but an alternative is to download the file to your computer and use scp to put the file on the router.