dfu: Delete superfluous initialization of the dfu_buf_size static variable

dfu: Delete superfluous initialization of the dfu_buf_size static variable

After extension of the dfu_get_buf() to also setup (implicitly) the dfu_buf_size
variable it is not needed to set dfu_buf_size to CONFIG_SYS_DFU_DATA_BUF_SIZE.

This variable is set in the dfu_get_buf() by not only considering
CONFIG_SYS_DFU_DATA_BUF but more importantly the "dfu_bufsiz" env variable.
Therefore, dfu_get_buf() should be used for initialization.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Przemyslaw Marczak <p.marczak@samsung.com>