Does disconnecting the Tx+ and Tx- wires in the ethernet cable make it Unidirectional for Packet (TCP) sniffing
Three things. 

Firstly, what layer are you doing the forwarding on? Without the ability to respond to ARP requests you will really need the switch to be doing dumb forwarding to a specified port at the ethernet level, I keep an old 10base-T switch around for this sort of thing. In particular the switch will not be able to discover your mac address and thus will not be able to figure out which port to forward to without help, most switches will also power down a port that does not have carrier present, so you will need to work around that.

Secondly, you probably need to hack the PHY (and maybe also the MAC) configuration registers to avoid the thing going to low power/port disconnected mode. A 100R resistor across the TX pair MAY help here, but it is going to depend a lot on the specific chip in play, download the chips datasheet and have plenty of coffee ready.

Thirdly, without ARP, the TCP stack will likely not do anything reasonable so you really want to be doing the interception at the ethernet frame level, not the higher levels of the stack.

In general this kind of 'data diode' is more trouble then it is worth, it can be done, but the easy way usually involves either custom network drivers or some very funky switch configuration, usually a small computer with three network interfaces (or two interfaces plus a high speed unidirectional port) is the easy way (And besides this way you find out which way a given frame is travelling), MITM has much to recommend it.