rgw/rgw_rest: do not set unused variable

rgw/rgw_rest: do not set unused variable

`len` is used to track the total length of received data, but we already
collect the data in `bl`, so we can always check `bl` for the total
length. this change also silences following warning from Clang:

../src/rgw/rgw_rest.cc:1477:21: warning: variable 'len' set but not used [-Wunused-but-set-variable]
  int read_len = 0, len = 0;
                    ^
1 warning generated.

Signed-off-by: Kefu Chai <kchai@redhat.com>