net: ipv4: Remove tcp_default_init_rwnd

This reverts commit 1e9a9ffb ("RFC: ANDROID: net: ipv4: tcp:
Namespace-ify sysctl_tcp_default_init_rwnd") and
commit 89e8c703 ("ANDROID: net: ipv4: tcp: add a sysctl to config
the tcp_default_init_rwnd")

This feature has been deprecated.

CRs-Fixed: 2500636
Change-Id: Ie831694ab897e2b267b8208833f5a2b28c1c2abe
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>