samples: net: zperf: Fix RAM overflow for nRF70

nRF70 driver now uses a separate heap, so, no need to explicitly
configure the kernel heap.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>

Author: krish2718

samples/net/zperf/boards/nrf7002dk_nrf5340_cpuapp.conf

@@ -4,7 +4,6 @@ CONFIG_NET_PKT_TX_COUNT=27
 CONFIG_NET_BUF_RX_COUNT=28
 CONFIG_NET_BUF_TX_COUNT=54
 CONFIG_NET_BUF_DATA_SIZE=1100
-CONFIG_HEAP_MEM_POOL_SIZE=260000
 CONFIG_NRF70_MAX_TX_AGGREGATION=4
 CONFIG_NRF70_QSPI_LOW_POWER=n