ICU-22421 Remove two unused internal methods in gregorian cal

Remove pinDayOfMonth() and yearLength(int32_t year)
from GregorianCalendar.

These two methods are
1. Unused by any code inside ICU, not in source/{common,i18n,test}.
2. Marked as @internal in the header.
3. Wrap inside #ifndef U_HIDE_INTERNAL_API block in the header
4. In "protected:" section.
5. No ICU4J counterpart.

The yearLength(int32_t year) dup the functionality as
handleGetYearLength of the same class and that one is the correct one to
be keep and used..
There is another yearLength() w/o the year as parameter should NOT be
removed and still needed internally.

icu4c/source/i18n/gregocal.cpp

@@ -610,14 +610,6 @@ GregorianCalendar::monthLength(int32_t month, int32_t year) const
 
 // -------------------------------------
 
-int32_t
-GregorianCalendar::yearLength(int32_t year) const
-{
-    return isLeapYear(year) ? 366 : 365;
-}
-
-// -------------------------------------
-
 int32_t
 GregorianCalendar::yearLength() const
 {
@@ -626,24 +618,6 @@ GregorianCalendar::yearLength() const
 
 // -------------------------------------
 
-/**
-* After adjustments such as add(MONTH), add(YEAR), we don't want the
-* month to jump around.  E.g., we don't want Jan 31 + 1 month to go to Mar
-* 3, we want it to go to Feb 28.  Adjustments which might run into this
-* problem call this method to retain the proper month.
-*/
-void 
-GregorianCalendar::pinDayOfMonth() 
-{
-    int32_t monthLen = monthLength(internalGetMonth());
-    int32_t dom = internalGet(UCAL_DATE);
-    if(dom > monthLen) 
-        set(UCAL_DATE, monthLen);
-}
-
-// -------------------------------------
-
-
 UBool
 GregorianCalendar::validateFields() const
 {

icu4c/source/i18n/unicode/gregocal.h

@@ -536,29 +536,13 @@ class U_I18N_API GregorianCalendar: public Calendar {
     virtual int32_t monthLength(int32_t month, int32_t year) const;
 
 #ifndef U_HIDE_INTERNAL_API
-    /**
-     * return the length of the given year.
-     * @param year    the given year.
-     * @return        the length of the given year.
-     * @internal
-     */
-    int32_t yearLength(int32_t year) const;
-
     /**
      * return the length of the year field.
      * @return    the length of the year field
      * @internal
      */
     int32_t yearLength(void) const;
 
-    /**
-     * After adjustments such as add(MONTH), add(YEAR), we don't want the
-     * month to jump around.  E.g., we don't want Jan 31 + 1 month to go to Mar
-     * 3, we want it to go to Feb 28.  Adjustments which might run into this
-     * problem call this method to retain the proper month.
-     * @internal
-     */
-    void pinDayOfMonth(void);
 #endif  /* U_HIDE_INTERNAL_API */
 
     /**