Remove libgmp-dev dependency

After merge of #1098 libsecp256k1 no longer uses
libgmp at all, so we have no need to include it
in our dependency list.

install.sh

@@ -46,7 +46,6 @@ deps_install ()
         'automake' \
         'pkg-config' \
         'libtool' \
-        'libgmp-dev' \
         'python3-dev' \
         'virtualenv' \
         'python3-pip' )