ovsdb: Remove duplicated function defintions

ovsdb: Remove duplicated function defintions

ovsdb_function_from_string() and ovsdb_function_to_string() are defined
both in ovsdb/condition.c and lib/ovsdb-condidtion.c with the same function
definition.  Remove the one in ovsdb/condition.c to avoid duplication.

This also resolves the following bazel building error.

./libopenvswitch.lo(ovsdb-condition.pic.o): In function `ovsdb_function_from_string':
/lib/ovsdb-condition.c:24: multiple definition of `ovsdb_function_from_string'
./libovsdb.a(condition.pic.o):/proc/self/cwd/external/openvswitch_repo/ovsdb/condition.c:34: first defined here
./libopenvswitch.lo(ovsdb-condition.pic.o): In function `ovsdb_function_from_string':
./lib/ovsdb-condition.c:24: multiple definition of `ovsdb_function_to_string'
./libovsdb.a(condition.pic.o):/proc/self/cwd/external/openvswitch_repo/ovsdb/condition.c:335

Reported-by: Harold Lim <haroldl@vmware.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>