Restrict domain transfer pricing to 1 year

This CL restricts domain transfer pricing lookups (on domain check and info) to
only support a 1-year period for inquiring about transfer fees.  That treatment
matches what we do for domain restores, which are also always one year.  This is
a followup to [] which disallowed actual transfer request flows from
specifying multi-year periods.

Since it's no longer necessary, this CL also changes the domain transfer pricing
logic to drop the years parameter, including removing the parameter from the
custom pricing logic TransferPriceParameters object.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150367839

docs/flows.md

@@ -576,6 +576,7 @@ information about the domain.
     *   The requested fees cannot be provided in the requested currency.
     *   Fee checks for command phases and subphases are not supported.
     *   Restores always renew a domain for one year.
+    *   Transfers always renew a domain for one year.
 
 ## DomainDeleteFlow
 
@@ -712,6 +713,7 @@ information.
     *   Domain labels cannot begin with a dash.
     *   Restores always renew a domain for one year.
     *   Domain labels cannot end with a dash.
+    *   Transfers always renew a domain for one year.
     *   Unknown fee command name.
     *   By server policy, fee check names must be listed in the availability
         check.