bgpd: retain peer asn even with remove-private-AS

bgpd: retain peer asn even with remove-private-AS

In situations where remove-private-AS is configured for eBGP peers
residing in a private ASN, the peer's ASN was not being retained
in the AS-Path which can allow loops to occur. This was addressed
in a prior commit but it only addressed cases where the "replace-AS"
keyword was configured.
This commit ensures we retain the peer's ASN when using
"remove-private-AS" for eBGP peers in a private ASN regardless of other
keywords.

Setup:
=========
router bgp 4200000002
 neighbor enp1s0 interface v6only remote-as external
 neighbor enp6s0 interface v6only remote-as external
 !
 address-family ipv4 unicast
  neighbor enp6s0 remove-private-AS
 exit-address-family

ub18# show ip bgp sum | include 420000
BGP router identifier 100.64.0.111, local AS number 4200000002 vrf-id 0    <<<<< local asn 4200000002
ub20(enp1s0)    4 4200000001        22        22        0    0    0 00:00:57            1        1
ub20(enp6s0)    4 4200000001        21        22        0    0    0 00:00:57            0        1   <<<< peer asn 4200000001

ub18# show ip bgp | include 0.2
Default local pref 100, local AS 4200000002
*> 100.64.0.2/32    enp1s0                   0             0 4200000001 4200000004 4200000005 4200000001 i

Before ("remote-private-AS" only):
=========
ub18# show ip bgp neighbors enp6s0 advertised-routes | include 100.64.0.2
*> 100.64.0.2/32    ::                                     0 i     <<<<<  empty as-path, no way to prevent loop

After ("remote-private-AS" only):
=========
ub18# show ip bgp neighbors enp6s0 advertised-routes | include 100.64.0.2
*> 100.64.0.2/32    ::                                     0 4200000001 4200000001 i    <<<< retain peer's asn, breaks loop

Ticket: 2857047
Signed-off-by: Trey Aspelund <taspelund@nvidia.com>