CLEANUP: mux-h2: Remove unused fields in h2c structures

CLEANUP: mux-h2: Remove unused fields in h2c structures

Some fields in h2c structures are not used: .mfl, .mft and .mff. Just remove
them.

.msi field is also removed. It is tested but never set, except when a H2
connection is initialized. It also means h2c_mux_busy() function is useless
because it always returns 0 (.msi is always -1). And thus, by transitivity,
H2_CF_DEM_MBUSY is also useless because it is never set. So .msi field,
h2c_mux_busy() function and H2C_MUX_BUSY flag are removed.