net: tftp: remove TFTP_MTU_BLOCKSIZE

Remove the unneeded define TFTP_MTU_BLOCKSIZE.
Since the KConfig migration done by commit b618b370 ("net:
Convert CONFIG_TFTP_BLOCKSIZE to Kconfig"), CONFIG_TFTP_BLOCKSIZE
is always defined and can be used directly to avoid confusion
(fallback to 1468 in code is never used).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>